1. Field of the Invention
The present invention relates to a process for producing a coil spring which has high strength and high fatigue resistance and which is used for a torsion spring of a clutch disc which may be a valve spring of an internal combustion engine for an automobile, a suspension spring for an automobile or a driving system part.
2. Description of the Related Art
It has been demanded for a coil spring, which may be a valve spring for an engine, a suspension spring for an automobile or a driving system part, to have high strength and high fatigue resistance so as to obtain high output of an engine for an automobile and to lighten a car body.
As the conventional method for producing a spring having high strength, the following method has been known. In the conventional method, after wire drawing a steel wire for spring, oil tempered wires for spring which hardening and tempering were conducted and which has high tensile strength were subjected to the coil molding; heat treatment; grinding; and remaining stress assigning by shot peening; and then polishing treatment was conducted so as to reduce the maximum roughness on the surface thereof.
For example, Japanese Unexamined Patent Publication (KOKAI) No. 4-247824 discloses a process for producing a spring having high strength as follows. In this process of the publication, the oil tempered wires for spring made of hardened and tempered steel which has high strength and whose tensile strength is not less than 1960N/mm.sup.2 are subjected to warm coiling at the temperature ranging from 100.degree. to 550.degree. C. To install this warm coiling machine used in this conventional process costs very much. Furthermore, even by using this warm coiling, it is difficult to mold a spring having D/d.ltoreq.4 in some cases.
Also, Japanese Unexamined Patent Publication (KOKAI) No. 5-179348 discloses a process for producing a coil spring having the same high strength and high fatigue resistance as those in the case of cold coiling as follows. In this process of the publication, coil forming is conducted by hot coiling the wire for cold coiling at the temperature ranging from 900.degree. to 1050.degree. C. and then, it is treated in tempering. It costs very much to install hot coiling machine which is necessary for this conventional process.
Furthermore, Japanese Unexamined Patent Publication No. 5-339763 discloses the following process as follows. In this conventional process, oil tempered wires for spring having oxide skin are subjected to coiling molding; heat treatment; descaling treatment so as to obtain the surface maximum roughness being not more than Rmax 5 .mu.m; nitriding treatment; and remaining stress assigning treatment by shot peening. Therefore, in this process, a surface polishing process is not necessary after the remaining stress assigning process.